Method of transmitting ack/nack signal in wireless communication system

ABSTRACT

A method of transmitting an acknowledgment (ACK)/non-acknowledgement (NACK) signal in a wireless communication system includes assigning at least one ACK channel among a plurality of ACK channels which share an ACK channel region for transmitting the ACK/NACK signal, and transmitting the ACK/NACK signal through the at least one ACK channel, wherein the ACK channel region includes at least one tile including a plurality of data subcarriers, and the ACK/NACK signal of each ACK channel is indicated by mapping different orthogonal vectors respectively to the plurality of ACK channels in the tile.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a 37 C.F.R. 81.53(b) continuation ofco-pending U.S. application Ser. No. 12/361,294 filed Jan. 28, 2009,which claims the benefit of U.S. Provisional Application No. 61/024,195filed on Jan. 28, 2008, and claims priority to Patent Application No.10-2008-0057272 filed in Republic of Korea on Jun. 18, 2008. The entirecontents of all of the above applications are hereby incorporated byreference.

FIELD OF THE INVENTION

The present invention relates to wireless communications, and moreparticularly, to a method for transmitting a multiplexed acknowledgment(ACK)/non-acknowledgement (NACK) signal by assigning a plurality of ACKchannels in one ACK channel region.

DESCRIPTION OF THE RELATED ART

The institute of electrical and electronics engineers (IEEE) 802.16standard provides a technique and protocol for supporting broadbandwireless access. The standardization had been conducted since 1999 untilthe IEEE 802.16-2001 was approved in 2001. The IEEE 802.16-2001 is basedon a physical layer of a single carrier (SC) called ‘WirelessMAN-SC’.The IEEE 802.16a standard was approved in 2003. In the IEEE 802.16astandard, ‘WirelessMAN-OFDM’ and ‘WirelessMAN-OFDMA’ are further addedto the physical layer in addition to the ‘WirelessMAN-SC’. Aftercompletion of the IEEE 802.16a standard, the revised IEEE 802.16-2004standard was approved in 2004. To correct bugs and errors of the IEEE802.16-2004 standard, the IEEE 802.16-2004/Cor1 was completed in 2005 ina format of ‘corrigendum’.

An error correction scheme is used to secure communication reliability.Examples of the error correction scheme include a forward errorcorrection (FEC) scheme and an automatic repeat request (ARQ) scheme. Inthe FEC scheme, errors in a receiving end are corrected by appending anextra error correction code to information bits. In the ARQ scheme,errors are corrected through data retransmission. Examples of the ARQscheme include a stop and wait (SAW) scheme, a go-back-N (GBN) scheme, aselective repeat (SR) scheme, etc. The SAW scheme is a scheme in which aframe is transmitted after determining whether the transmitted frame iscorrectly received. The GBN scheme is a scheme in which N contiguousframes are transmitted, and if transmission is unsuccessful, all frameswhich are transmitted after an erroneous frame are retransmitted. The SRscheme is a scheme in which only the erroneous frame is selectivelyretransmitted.

The FEC scheme has an advantage in that a time delay is not significantand no information is additionally exchanged between a transmitting endand the receiving end but also has a disadvantage in that systemefficiency deteriorates in a good channel environment. The ARQ schemehas an advantage in that transmission reliability can be increased butalso has a disadvantage in that a time delay occurs and systemefficiency deteriorates in a poor channel environment. To solve suchdisadvantages, a hybrid automatic repeat request (HARQ) scheme isproposed by combining the FEC scheme and the ARQ scheme. In the HARQscheme, it is determined whether an unrecoverable error is included indata received by a physical, and retransmission is requested upondetecting the error, thereby improving performance.

In the HARQ scheme, if no error is detected from the received data, areceiver transmits an acknowledgement (ACK) signal as a response signalso that a transmitter is informed that the data is successfullyreceived. Otherwise, if an error is detected from the received data, thereceiver transmits a non-acknowledgement (NACK) signal as a responsesignal so that the transmitter is informed that the data isunsuccessfully received. When the NACK signal is received, thetransmitter transmits retransmission data. The ACK/NACK signal is animportant control signal for ensuring communication reliability and isfrequently transmitted according to the number of data transmissionattempts.

Accordingly, there is a need for a method capable of effectivelytransmitting an ACK/NACK signal frequently transmitted in a wirelesscommunication system.

SUMMARY OF THE INVENTION

The present invention provides a method for transmitting anacknowledgment (ACK)/non-acknowledgement (NACK) signal in a wirelesscommunication system.

In an aspect, a method of transmitting an ACK/NACK signal in a wirelesscommunication system includes assigning at least one ACK channel among aplurality of ACK channels which share an ACK channel region fortransmitting the ACK/NACK signal, and transmitting the ACK/NACK signalthrough the at least one ACK channel, wherein the ACK channel regionincludes at least one tile including a plurality of data subcarriers,and the ACK/NACK signal of each ACK channel is indicated by mappingdifferent orthogonal vectors respectively to the plurality of ACKchannels in the tile.

In another aspect, a method of transmitting an ACK/NACK signal in awireless communication system includes receiving downlink data, andtransmitting the ACK/NACK signal through an ACK channel in response tothe downlink data, wherein the ACK/NACK signal is expressed with anorthogonal vector selected from a plurality of orthogonal vector setsrespectively corresponding to a plurality of ACK channels iterativelyassigned to one ACK channel region.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wireless communication system.

FIG. 2 shows an example of a frame structure.

FIG. 3 shows an example of a tile.

FIG. 4 shows an example of a fast-feedback region.

FIG. 5 shows assignment of a plurality of acknowledgment (ACK) channelsin an ACK channel region according to an embodiment of the presentinvention.

FIG. 6 shows a pilot allocation method in an ACK channel to bemultiplexed according to an embodiment of the present invention.

FIG. 7 shows a pilot allocation method in an ACK channel to bemultiplexed according to another embodiment of the present invention.

FIG. 8 shows a tile structure according to an embodiment of the presentinvention.

FIG. 9 is a flow diagram showing a data transmission method using hybridautomatic repeat request (HARQ) according to an embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a wireless communication system. The wireless communicationsystem can be widely deployed to provide a variety of communicationservices, such as voices, packet data, etc.

Referring to FIG. 1, the wireless communication system includes at leastone user equipment (UE) 10 and a base station (BS) 20. The UE 10 may befixed or mobile, and may be referred to as another terminology, such asa mobile station (MS), a user terminal (UT), a subscriber station (SS),a wireless device, etc. The BS 20 is generally a fixed station thatcommunicates with the UE 10 and may be referred to as anotherterminology, such as a node-B, a base transceiver system (BTS), anaccess point, etc. There are one or more cells within the coverage ofthe BS 20.

A downlink (DL) represents a communication link from the BS 20 to the UE10, and an uplink (UL) represents a communication link from the UE 10 tothe BS 20. In the DL, a transmitter may be a part of the BS 20, and areceiver may be a part of the UE 10. In the UL, the transmitter may be apart of the UE 10, and the receiver may be a part of the BS 20.

The wireless communication system may be an orthogonal frequencydivision multiplexing (OFDM)/orthogonal frequency division multipleaccess (OFDMA)-based system. The OFDM uses a plurality of orthogonalsubcarriers. Further, the OFDM uses an orthogonality between inversefast Fourier transform (IFFT) and fast Fourier transform (FFT). Thetransmitter transmits data by performing IFFT. The receiver restoresoriginal data by performing FFT on a received signal. The transmitteruses IFFT to combine the plurality of subcarriers, and the receiver usesFFT to split the plurality of subcarriers.

FIG. 2 shows an example of a frame structure. A frame is a data sequenceused according to a physical specification in a fixed time duration.This may be found in section 8.4.4.2 of the “Part 16: Air Interface forFixed Broadband Wireless Access Systems” in the institute of electricaland electronics engineers (IEEE) standard 802.16-2004 (hereinafter,Document 1).

Referring to FIG. 2, the frame includes a downlink (DL) frame and anuplink (UL) frame. In a time division duplexing (TDD) scheme, UL and DLtransmissions are achieved at different time points but share the samefrequency band. The DL frame temporally precedes the UL frame. The DLframe sequentially includes a preamble, a frame control header (FCH), aDL-MAP, a UL-MAP, and a burst region. Guard times are provided toidentify the UL frame and the DL frame and are inserted to a middleportion (between the DL frame and the UL frame) and a last portion (nextto the UL frame) of the frame. A transmit/receive transition gap (TTG)is a gap between a downlink burst and a subsequent uplink burst. Areceive/transmit transition gap (RTG) is a gap between an uplink burstand a subsequent downlink burst.

A preamble is used between a BS and a UE for initial synchronization,cell search, and frequency-offset and channel estimation. The FCHincludes information on a length of a DL-MAP message and a coding schemeof the DL-MAP.

The DL-MAP is a region for transmitting the DL-MAP message. The DL-MAPmessage defines access to a downlink channel. The DL-MAP messageincludes a configuration change count of a downlink channel descriptor(DCD) and a BS identifier (ID). The DCD describes a downlink burstprofile applied to a current MAP. The downlink burst profile indicatescharacteristics of a downlink physical channel. The DCD is periodicallytransmitted by the BS by using a DCD message.

The UL-MAP is a region for transmitting a UL-MAP message. The UL-MAPmessage defines access to an uplink channel. The UL-MAP message includesa configuration change count of an uplink channel descriptor (UCD) andalso includes an effective start time of uplink allocation defined bythe UL-MAP. The UCD describes an uplink burst profile. The uplink burstprofile indicates characteristics of an uplink physical channel and isperiodically transmitted by the BS by using a UCD message.

A fast-feedback region is included in a portion of the UL frame. Thefast-feedback region is a region which is assigned for faster uplinktransmission than general uplink data. Channel quality information (CQI)or acknowledgement (ACK)/non-acknowledgement (NACK) signals can becarried on the fast-feedback region. The fast-feedback region may belocated in any position in the UL frame, and the position or size of thefast-feedback region is not limited as shown in FIG. 2.

Hereinafter, a slot is a minimum unit of possible data allocation, andis defined with a time and a subchannel. The number of subchannelsdepends on an FFT size and time-frequency mapping. Each subchannelincludes a plurality of subcarriers. The number of subcarriers includedin each subchannel differs according to a permutation rule. Permutationdenotes mapping from a logical subchannel to a physical subcarrier. Infull usage of subchannels (FUSC), each subchannel includes 48subcarriers. In partial usage of subchannels (PUSC), each subchannelincludes 24 or 16 subcarriers. A segment denotes at least one subchannelset.

In order for data to be mapped to physical subcarriers in a physicallayer, two steps are generally performed on the data. In a first step,the data is mapped to at least one data slot on at least one logicalsubchannel. In a second step, each logical subchannel is mapped to aphysical subcarrier. This is called permutation. Examples of thepermutation rule employed in the Document 1 above include FUSC, PUSC,optional-FUSC (O-FUSC), optional-PUSC (O-PUSC), adaptive modulation andcoding (AMC), etc. A set of OFDM symbols using the same permutation ruleis referred to as a permutation zone. One frame includes at least onepermutation zone.

The FUSC and the O-FUSC are used only in downlink transmission. The FUSCconsists of one segment including all subchannel groups. Each subchannelis mapped to a physical subcarrier distributed over the entire physicalchannel. This mapping varies for each OFDM symbol. A slot consists ofone subchannel on one OFDM symbol. The O-FUSC uses a pilot allocationscheme different from that used in the FUSC.

The PUSC is used both in downlink transmission and uplink transmission.In downlink, each physical channel is divided into clusters, each ofwhich includes 14 contiguous subcarriers on two OFDM symbols. Thephysical channel is mapped to six groups. In each group, pilots areallocated in fixed positions to each cluster. In uplink, subcarriers aredivided into tiles, each of which includes four contiguous physicalsubcarriers on three OFDM symbols. The subchannel includes six tiles.Pilots are allocated to the corners of each tile. The O-PUSC is usedonly in uplink transmission. Each tile includes three contiguousphysical subcarriers on three OFDM symbols. Pilots are allocated to thecenter of each tile.

FIG. 3 shows an example of a tile. The tile is a PUSC tile.

Referring to FIG. 3, one tile consists of 4 subcarriers in a frequencydomain and 3 OFDM symbols in a time domain, that is, 12 subcarriers intotal. The 12 subcarriers can be divided into 8 data subcarriers M₀ toM₇ and 4 pilot subcarriers. A data symbol is carried on the datasubcarrier. The data symbol may be a symbol for user data or a controlsignal. A pilot or ‘Null’ can be carried on the pilot subcarrier. Toutilize the pilot subcarrier, other data symbols may be carried andtransmitted on the pilot subcarrier. In uplink, a subchannel of the PUSCpermutation rule includes 48 data subcarriers and 24 pilot subcarriers.

FIG. 4 shows an example of a fast-feedback region.

Referring to FIG. 4, a fast-feedback message is mapped to thefast-feedback region. One fast-feedback message can occupy onefast-feedback slot. Herein, it is shown that four fast-feedback slotseach having a size of 3 OFDM symbols are assigned to one fast-feedbackregion. The fast-feedback slot may correspond to one subchannel.

One subchannel may include a plurality of tiles. For clarity ofexplanation, it is assumed that one subchannel includes 6 tiles. In thePUSC permutation rule, one subchannel includes 48 data subcarriers and24 pilot subcarriers. In the O-PUSC permutation rule, one subchannelincludes 48 data subcarriers and 6 pilot subcarriers. Six tiles can bedistributively located throughout the entire band.

The fast-feedback region can be assigned to an ACK channel. The ACKchannel is a channel for transmitting an ACK/NACK signal. A ½ subchannel(i.e., 3 tiles) can be assigned to the ACK channel. The ACK channel canuse the PUSC or the O-PUSC permutation rule.

The ACK/NACK signal can be mapped to a data subcarrier of each tile.Table 1 below shows modulation symbols carried on the 8 data subcarriersincluded in one tile. One modulation symbol is carried on one datasubcarrier, and 8 modulation symbols carried on one tile constitute onevector. 8 types of vector can be formed in total, and their indices havevalues in the range of 0 to 7.

TABLE 1 Vector Index M

, M_(n,)

− 1, ... M_(n,)

− 7 0 P0, P1, P2, P3, P0, P1, P2, P3 1 P0, P3, P2, P1, P0, P3, P2, P1 2P0, P0, P1, P1, P2, P2, P3, P3 3 P0, P0, P3, P3, P2, P2, P1, P1 4 P0,P0, P0, P0, P0, P0, P0, P0 5 P0, P2, P0, P2, P0, P2, P0, P2 6 P0, P2,P0, P2, P2, P0, P2, P0 7 P0, P2, P2, P0, P2, P0, P0, P2

indicates data missing or illegible when filed

Symbols constituting each vector can be expressed by Equation 1 below.P(0), P(1), P(2), and P(3) denote phases of symbols on a constellationmap. This can be used in a quadrature phase shift keying (QPSK)modulation scheme.

$\begin{matrix}{{{P\; 0} = {\exp \left( {j \cdot \frac{\pi}{4}} \right)}}{{P\; 1} = {\exp \left( {j \cdot \frac{3\pi}{4}} \right)}}{{P\; 2} = {\exp \left( {{- j} \cdot \frac{3\pi}{4}} \right)}}{{P\; 3} = {\exp \left( {{- j} \cdot \frac{\pi}{4}} \right)}}} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

Herein, vectors having different indices are orthogonal to one another.

If 1-bit payload represents the ACK/NACK signal (e.g., if 0, theACK/NACK signal is ACK and if 1, the ACK/NACK signal is NACK), a vectorassigned to the ACK channel can be expressed by Table 2 below.

TABLE 2 Vector indices per Tile ACK 1-bit symbol Tile(0), Tile(1),Tile(2) 0 0, 0, 0 1 4, 7, 2

A ½ subchannel is assigned for the ACK channel, and thus three tiles areassigned to one ACK channel. Since one vector is assigned to one tile,three vectors are required to express the ACK/NACK signal. An ACK/NACKsignal mapped to the ACK channel and expressed in a vector format isreferred to as a codeword of the ACK/NACK signal.

Mapping of the ACK/NACK signal on the ACK channel may be found in sector8.4.5.4.13 of the IEEE standard 802.16-2004 “Part 16: Air Interface forFixed Broadband Wireless Access Systems”.

Meanwhile, a transmitter that performs HARQ can transmit m transmissionblocks through one frame by performing a plurality of HARQ processes ina parallel manner, and a receiver can transmit m ACK/NACK signals inresponse to the m transmission blocks (where m is integer greater than1). In addition, a BS that transmits data for a plurality of usersreceives a plurality of ACK/NACK signals from the plurality of users.When one ACK channel (i.e., ½ subchannel) is assigned for one ACK/NACKsignal in a system in which a plurality of ACK/NACK signals aretransmitted according to the HARQ, limited radio resources can bewasted. For example, if the BS transmits 10 transmission blocks to oneuser, the BS has to assign 5 subchannels to one user to receive theACK/NACK signals. Alternatively, if the BS transmits one transmissionblock to each of 10 users, the BS also has to assign 5 subchannels toreceive the ACK/NACK signals from the 10 users. The number of ACK/NACKsignals is increased in proportion to the number of pieces of data to betransmitted. Accordingly, an amount of radio resources used for theACK/NACK signals is also increased. Due to limited radio resources,there is a limit in allocation of radio resources that can be allocatedfor the ACK/NACK signals.

A method capable of transmitting a plurality of ACK/NACK signals in amore effective manner is necessary. For this, instead of assigning oneACK channel to one ACK/NACK signal, the BS assigns one ACK channel tothe plurality of ACK/NACK signals. The plurality of ACK/NACK signals maybe ACK/NACK signals of multiple users or ACK/NACK signals of a singleuser. That is, the BS may assign one ACK channel to the multiple usersso that the ACK/NACK signals of the multiple users are multiplexed toone ACK channel, or may assign one ACK channel to the single user sothat the single user can perform transmission by multiplexing theplurality of ACK/NACK signals to one ACK channel.

Hereinafter, a method capable of multiplexing and transmitting aplurality of ACK/NACK signals will be described.

<Multiplexing of ACK/NACK Signal by Using Pilot>

A plurality of ACK channels are assigned to an ACK channel region fortransmitting an ACK/NACK signal and thus a multiplexed ACK/NACK signalis transmitted. That is, a plurality of ACK channels share one ACKchannel region. It is assumed hereinafter that one ACK channel uses a ½subchannel (i.e., 3 tiles) of the PUSC permutation rule. This is forexemplary purposes only, and thus the number of tiles and thepermutation rule used in the ACK channel are not limited thereto.

FIG. 5 shows assignment of a plurality of ACK channels in an ACK channelregion according to an embodiment of the present invention.

Referring to FIG. 5, an ACK channel (ACKCH) region is a region where anACKCH for transmitting an ACK/NACK signal is assigned in a frame. TheACKCH region may be a fast-feedback region. A ½ slot of thefast-feedback region can be assigned to the ACKCH region. That is, a ½subchannel can be assigned to the ACKCH region. Although one ACKCHregion is shown herein, a plurality of ACKCH regions can be assigned inthe frame.

The ACK/NACK signal is multiplexed by assigning a plurality of ACKCHs toone ACKCH region. Since the ACKCH uses the ½ subchannel of the PUSCpermutation rule, 3 tiles are assigned to one ACKCH. Each channelincludes 8 data subcarriers and 4 pilot subcarriers. The ACKCHs assignedto one ACKCH region can be iteratively assigned by the number of pilotsubcarriers of a tile. That is, if a channel is estimated using a pilot,ACKCHs can be iteratively assigned by the number of pilots.

Since one tile includes 4 pilots, 4 ACKCHs can be multiplexed in oneACKCH region. The BS can assign a specific number of ACKCHs to one ACKCHregion, where the specific number corresponds to the number of pilotsubcarriers. The BS can assign 4 ACKCHs to the ACKCH region that usesthe PUSC permutation rule. The ACKCH multiplexed in one ACKCH region canbe assigned to one UE or a plurality of UEs.

A plurality of ACK/NACK signals are multiplexed to a plurality of ACKCHsassigned to one ACKCH region by using an orthogonal vector set. Table 3below shows an example of vector sets assigned to the ACKCHs.

TABLE 3 vector indices per tile ACK Channel ACK/NACK signal (OrthogonalVector sets) ACKCH 1 ACK 0, 0, 0 NACK 4, 7, 2 ACKCH 2 ACK 1, 1, 1 NACK2, 2, 7 ACKCH 3 ACK 3, 3, 3 NACK 5, 5, 5 ACKCH 4 ACK 6, 6, 6 NACK 7, 4,4

In Table 3, a vector index indicates a vector consisting of 8 modulationsymbols. As a vector used for each ACKCH, 8 vectors consisting of 8modulation symbols can be used as shown in Table 1. 3 vectors mayconstitute one vector set, and may constitute 8 types of mutuallyorthogonal vector sets. Since the 8 types of vector sets used in eachACKCH are orthogonal, interference does not occur between the vectorsets. Therefore, the ACK/NACK signal can be multiplexed to 4 ACKCHs bythe using mutually orthogonal vector sets.

If a plurality of ACKCHs are assigned to one ACKCH region, a pilot maybe used to identify each ACKCH. That is, a plurality of ACK/NACK signalstransmitted through one ACKCH region can be identified by using a pilot.A method for identifying a multiplexed ACK/NACK signal by using thepilot includes a pilot separation method and an orthogonal pilotallocation method.

First, the pilot separation method will be described.

FIG. 6 shows a pilot allocation method in an ACKCH to be multiplexedaccording to an embodiment of the present invention. 4 ACKCHs 1, 2, 3,and 4 are assigned to one ACKCH region, and only one tile of each ACKCHis shown herein.

Referring to FIG. 6, the pilot separation method is a method in whichpilots of a tile are used by dividing the pilots by the number of ACKCHsassigned to one ACKCH region. If 4 ACKCHs are assigned to one ACKCHregion, any one of 4 pilot subcarriers of the tile is used in eachACKCH. That is, one ACKCH performs transmission by carrying a pilot P ononly any one of the 4 pilot subcarriers, and transmits null subcarriersby carrying NULL (N) on the remaining pilot subcarriers. In this case, apilot of each ACKCH is assigned to a pilot subcarrier located at aposition where the ACKCHs do not overlap with each another.

The position of the pilot subcarrier used for pilot transmission can bepredetermined for each ACKCH. The BS can report the position of thepilot subcarrier, on which a pilot is transmitted, to the UE for eachACKCH while assigning a plurality of ACKCHs to one ACKCH region. The BScan estimate a channel state of the plurality of ACKCHs by using a pilotcarried on each pilot subcarrier. Since 4 pilots are used, the channelstate can be estimated for 4 ACKCHs. A multiplexed ACK/NACK signaltransmitted by being carried on a data subcarrier can be decoded byconsidering the estimated channel state.

Although it has been described herein that 4 ACKCHs are assigned to oneACKCH region, the number of ACKCHs that can be iteratively assigned toone ACKCH region can be variously determined to be a value less than orequal to the number of pilot subcarriers of a tile, and there is norestriction on the position and number of pilot subcarriers used forpilot transmission in each ACKCH.

Now, the orthogonal pilot allocation method will be described.

FIG. 7 shows a pilot allocation method in an ACKCH to be multiplexedaccording to another embodiment of the present invention. 4 ACKCHs 1, 2,3, and 4 are assigned to one ACKCH region, and only one tile of eachACKCH is shown herein.

Referring to FIG. 7, the orthogonal pilot allocation method is a methodin which pilots having different patterns are assigned to each ACKCH sothat the pilots do not interfere with one another among a plurality ofACKCHs assigned to one ACKCH region. If 4 ACKCHs are assigned to oneACKCH region, 4 pilot patterns are used. A first ACKCH carries andtransmits pilots only on two pilot subcarriers among 4 pilotsubcarriers, and transmits null subcarriers by carrying NULL (N) on theremaining two pilot subcarriers. A second ACKCH carries and transmitspilots on two pilot subcarriers located at a position where the secondACKCH does not overlap with the first ACKCH, and transmits the remainingtwo pilots as null subcarriers. A third ACKCH carries and transmits apilot whose phase is shifted to (+) on one pilot subcarrier among the 4pilot subcarriers, carries and transmits a pilot whose phase is shiftedto (−) on another pilot subcarrier, and transmits null subcarriers bycarrying NULL on the remaining two pilot subcarriers. Herein, (+) and(−) denote phase modulation of a symbol on a constellation map. A pilotphase may be modulated to identify two types of signals according to abinary phase shift keying (BPSK) scheme. A fourth ACKCH carries a pilotwhose phase is shifted to (+) on one pilot subcarrier between two pilotsubcarriers used as null subcarriers in the third ACKCH, carries andtransmits a pilot whose phase is shifted to (−) on the other pilotsubcarrier, and transmits the remaining two pilot subcarriers as nullsubcarriers.

A phase modulation value and a position of a pilot subcarrier used forpilot transmission can be predetermined for each ACKCH. The BS canreport a pilot pattern of the ACKCH to the UE while assigning aplurality of ACKCHs to one ACKCH region. The BS can estimate a channelstate according to the pilot pattern of each ACKCH, and can decode amultiplexed ACK/NACK signal carried and transmitted on a data subcarrierby considering the estimated channel state.

The number of ACKCHs that can be iteratively assigned to one ACKCHregion can be determined variously according to the pilot pattern, andan orthogonal pilot pattern can be configured variously.

<Multiplexing of ACK/NACK Signal by not Using Pilot>

A plurality of ACKCHs are assigned to an ACKCH region for transmittingan ACK/NACK signal and thus a multiplexed ACK/NACK signal istransmitted. In this case, a tile used by the ACKCH does not include apilot subcarrier, and all subcarriers are used as data subcarriers.

FIG. 8 shows a tile structure according to an embodiment of the presentinvention.

Referring to FIG. 8, an orthogonal code is used as an ACK/NACK signal.If the orthogonal code is used as the ACK/NACK signal, a plurality ofACK/NACK signals can be decoded without having to estimate a channelstate by using a pilot.

Assume that 3 tiles are used in an ACKCH region, and each tile consistsof 4 subcarriers on 3 OFDM symbols in a frequency domain. Instead ofassigning a pilot subcarrier to each tile, all of 12 subcarriers areassigned to data subcarriers M₀, M₁, . . . , and M₁₁. An orthogonal codehaving a length of 12 is mapped to each tile. 3 orthogonal codes mappedto 3 tiles represent one ACK/NACK signal. Examples of the orthogonalcode may include a Hadamard code, a discrete Fourier transform (DFT)sequence, a Walsh code, a constant amplitude zero auto-correlation(CAZAC) sequence, etc.

Equation 2 shows an example of a DFT matrix.

$\begin{matrix}{{\begin{bmatrix}W_{1} \\W_{2} \\\vdots \\W_{N}\end{bmatrix} = \begin{bmatrix}1 & 1 & 1 & \ldots & 1 \\1 & w & w^{2} & \ldots & w^{N - 1} \\\vdots & \vdots & \vdots & \; & \vdots \\1 & w^{N - 1} & w^{2{({N - 1})}} & \ldots & w^{{({N - 1})}{({N - 1})}}\end{bmatrix}}{{where},{w = ^{{- {j2\pi}}/N}}}} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack\end{matrix}$

If N=12, 12 DFT sequences having a length of 12 can be generated. The 12DFT sequences can be expressed by Equation 3 below.

W _(n) =[w ^(0·(n-1)) w ^(1·(n-1)) . . . w ^(11·(n-1))]  [Equation 3]

Herein, an index n of a DFT sequence has a value in the range of 1 to12. A vector set that represents an ACK/NACK signal can be configured byusing a DFT sequence having a length of 12. Table 4 shows an example ofa vector set using the DFT sequence.

TABLE 4 orthogonal code per tile ACK Channel ACK/NACK signal (Orthogonalcode Vector sets) ACKCH 1 ACK W₁, W₁, W₁ NACK W₂, W₂, W₂ ACKCH 2 ACK W₃,W₃, W₃ NACK W₄, W₄, W₄ ACKCH 3 ACK W₅, W₅, W₅ NACK W₆, W₆, W₆ ACKCH 4ACK W₇, W₇, W₇ NACK W₈, W₈, W₈ ACKCH 5 ACK W₉, W₉, W₉ NACK W₁₀, W₁₀, W₁₀ACKCH 6 ACK W₁₁, W₁₁, W₁₁ NACK W₁₂, W₁₂, W₁₂

When the vector set is configured using 12 DFT sequences, 6 ACKCHs canbe assigned to one ACKCH region. The 12 DFT sequences representrespective ACK/NACK signals of 6 ACKCHs. 3 DFT sequences arerespectively assigned to 3 tiles of the ACKCH.

Although the DFT sequence is used herein as an example, the vectorrepresenting the ACK/NACK signal of each ACKCH can be configured usingvarious orthogonal codes such as the Hadamard code, the Walsh code, theCAZAC sequence, etc. The ACKCH dose not have to use 3 tiles always, andone ACK/NACK signal can be expressed by assigning one orthogonal code toone tile. In addition, when the ACKCH uses a tile of the O-PUSCpermutation rule, 9 vector sets can be configured using an orthogonalcode having a length of 9 so that mapping can be achieved to 9subcarriers included in the tile. 5 ACKCHs can be assigned to one ACKCHregion by using the 9 vector sets. The ACK/NACK signal can be expressedwith a vector set in 4 ACKCHs, and only one ACK signal can be expressedas a vector set in a 5^(th) ACKCH,

When the ACKCH signal expressed with the orthogonal code is transmitted,the BS can obtain an index of the orthogonal code by using a correlationvalue of the orthogonal code. According to the index of the orthogonalcode, the BS can know to which ACKCH a received signal (i.e., an ACKsignal or a NACK signal) belongs among the plurality of ACKCHs.

FIG. 9 is a flow diagram showing a data transmission method using hybridautomatic repeat request (HARQ) according to an embodiment of thepresent invention.

Referring to FIG. 9, a BS transmits downlink data (step S110). A UEreceives the downlink data, and decodes the received downlink data todetect an error (step S120). As a response signal for the downlink data,the UE transmits an ACK/NACK signal (step S130). If no error is detectedfrom the downlink data, the UE transmits an ACK signal. If the error isdetected from the downlink data, the UE transmits a NACK signal.

While or before downlink data is transmitted, the BS allocates radioresources on which the UE transmits the ACK/NACK signal through adownlink control channel. In this case, the BS can assign one ACKCHregion to a plurality of UEs. That is, the ACK/NACK signal transmittedfrom the plurality of UEs is multiplexed and transmitted in one ACKCHregion. Alternatively, the BS can assign to one UE a plurality of ACKCHsassigned to one ACKCH region. The UE can transmit a plurality ofACK/NACK signals through each ACKCH. The plurality of ACK/NACK signalstransmitted from a plurality of UEs or one UE can be multiplexed usingthe aforementioned pilot separation method, orthogonal pilot allocationmethod, orthogonal code vector set, etc. An ACK/NACK signal transmissionmethod used by the UE may be a predetermined method or may be determinedby the BS. The BS can identify the multiplexed ACK/NACK signal by usinga correlation value of an orthogonal code or a pilot.

Although downlink HARQ has been described above, the technical featuresof the present invention can equally apply to uplink HARQ. In addition,the present invention is not limited to the aforementioned tilestructure, and thus the position and number of pilot subcarriers anddata subcarriers included in a tile can be changed variously.

Since a multiplexed acknowledgment (ACK)/non-acknowledgement (NACK)signal can be transmitted by assigning a plurality of ACK channels inone ACK channel region, limited radio resources can be effectively used.

Every function as described above can be performed by a processor suchas a microprocessor based on software coded to perform such function, aprogram code, etc., a controller, a micro-controller, an ASIC(Application Specific Integrated Circuit), or the like. Planning,developing and implementing such codes may be obvious for the skilledperson in the art based on the description of the present invention.

Although the embodiments of the present invention have been disclosedfor illustrative purposes, those skilled in the art will appreciate thatvarious modifications, additions and substitutions are possible, withoutdeparting from the scope of the invention. Accordingly, the embodimentsof the present invention are not limited to the above-describedembodiments but are defined by the claims which follow, along with theirfull scope of equivalents.

What is claimed is:
 1. A method of transmitting an acknowledgment(ACK)/non-acknowledgement (NACK) signal in a wireless communicationsystem, the method comprising: assigning at least one ACK channelamongst a plurality of ACK channels which share an ACK channel regionfor transmitting the ACK/NACK signal; and transmitting the ACK/NACKsignal through one of the at least one ACK channel, wherein each of theat least one ACK channel includes repeated tiles, wherein each of therepeated tiles is arranged with a plurality of symbols in a time domainand a plurality of subcarriers in a frequency domain, and includes acontinuous sequence of data subcarriers that does not include pilotsubcarriers, and wherein the ACK/NACK signal is assigned an orthogonalcode having a length proportional to a number of data subcarriers in thecontinuous sequence of data subcarriers.
 2. The method of claim 1,wherein the orthogonal code is a Walsh code.
 3. The method of claim 1,wherein the number of the repeated tiles included in one ACK channel isset to
 3. 4. The method of claim 1, wherein the ACK/NACK signalrepresents one of acknowledgment or acknowledgment based on the assignedorthogonal code.
 5. The method of claim 1, wherein the number of datasubcarriers in the continuous sequence of data subcarriers is equal to anumeric value derived by multiplying a number of the plurality ofsymbols in the time domain with a number of the plurality of subcarriersin the frequency domain.
 6. A transmitter configured to transmit anacknowledgment (ACK)/non-acknowledgement (NACK) signal in a wirelesscommunication system, the transmitter comprising: a processor configuredto assign at least one ACK channel amongst a plurality of ACK channelswhich share an ACK channel region for transmitting the ACK/NACK signal;and a radio frequency unit configured to transmit the ACK/NACK signalthrough one of the at least one ACK channel, wherein each of the atleast one ACK channel includes repeated tiles, wherein each of therepeated tiles is arranged with a plurality of symbols in a time domainand a plurality of subcarriers in a frequency domain, and includes acontinuous sequence of data subcarriers that does not include pilotsubcarriers, and wherein the ACK/NACK signal is assigned an orthogonalcode having a length proportional to a number of data subcarriers in thecontinuous sequence of data subcarriers.
 7. The transmitter of claim 6,wherein the orthogonal code is a Walsh code.
 8. The transmitter of claim6, wherein the number of the repeated tiles included in one ACK channelis set to
 3. 9. The transmitter of claim 6, wherein the ACK/NACK signalrepresents one of acknowledgment or acknowledgment based on the assignedorthogonal code.
 10. The transmitter of claim 6, wherein the number ofdata subcarriers in the continuous sequence of data subcarriers is equalto a numeric value derived by multiplying a number of the plurality ofsymbols in the time domain with a number of the plurality of subcarriersin the frequency domain.